1. Field of the invention
The present invention relates to a rear fork attachment structure for motorcycles.
2. Description of Related Art
Many motorcycles have a rear wheel suspension device including a rear fork (corresponding to a swing arm) for a rear wheel which is vertically swingably mounted on a vehicle body frame, and a rear suspension by which the rear fork is suspended from the vehicle body frame. One such rear wheel suspension device is known from Japanese utility model publication No. 62-60491 entitled "Swing arm attachment structure for motorcycles", for example.
According to the above related art, as shown in FIG. 1 of the above publication, an attachment 4 of a swing arm extends between right and left ends 1', 1 of a frame having a predetermined width, and the right and left ends 1' and 1 and the attachment 4 are coupled together by a pivot bolt 11 (the reference numerals shown herein and below are the same as those in the publication).
According to the above related art, a collar 8 held against an inner end surface of the right end 1', bearings 9, and a collar 7 are fitted over the pivot bolt 11, and fastened together with the right and left ends 1', 1 by the pivot bolt 11 and a nut 13. Since these parts 7 through 9, right and left ends 1', 1, and the attachment 4 have manufacturing tolerances, it is necessary to provide a clearance in view of an assembling tolerance between the right and left ends 1', 1. In order to assemble the parts 7 through 9 between the right and left ends 1', 1 without any clearance, an adjustment collar 10 is employed to make axial adjustments along the pivot bolt 11. Since there is no clearance, when the pivot bolt 11 and the nut 13 are tightened together, any bending moments due to the tightening forces do not act on the right and left ends 1', 1.
An engine is integrally mounted on the frame and has an engine case which is extremely rigid as is well known in the art. If an extremely rigid case is used, then it can increase the rigidity of a portion of the frame on which a rear fork is mounted. For example, the rear fork is supported by the engine case and the right and left ends of the frame. Consequently, the rigidity with which to support the rear fork is higher than if the rear fork is supported by only the right and left ends 1', 1 of the frame as is the case with the prior art.
However, unless both a clearance between the rear fork and the engine case and a clearance between the rear fork and the frame are eliminated, when the pivot bolt 11 and the nut 13 are tightened together, bending moments due to the tightening forces act on the rear fork and the frame. Therefore, it is necessary to take some measures against the application of such bending moments.